3.3.3Movement and Interactions
Making Salts — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 9 parts Lesson 36 of 59
Section 1 of 9Choosing a method
| Starting materials | Method | Example |
|---|---|---|
| Insoluble base, carbonate or reactive metal + acid | Add excess solid to acid, then filter off the extra | Copper oxide + sulfuric acid → copper sulfate |
| Soluble base (alkali) + acid | Titration to find the exact volumes, then evaporate | Sodium hydroxide + hydrochloric acid → sodium chloride |
| Insoluble salt | Precipitation: mix two soluble solutions and filter | Lead nitrate + sodium sulfate → lead sulfate |
Section 2 of 9Solubility rules
Always soluble
- Always soluble All sodium, potassium and ammonium salts, and all nitrates.
- These rules let you decide quickly whether a salt can be made by filtering, titration or precipitation.
Mostly soluble
- Mostly soluble Chlorides, except silver and lead chlorides.
- Sulfates, except lead, barium and calcium sulfates.
Mostly insoluble
- Mostly insoluble Carbonates and hydroxides, except those of sodium, potassium and ammonium.
Section 3 of 9Preparing a soluble salt from an insoluble base
1
Warm the dilute acid gently in a beaker.
2
Add the solid base a little at a time, stirring, until no more dissolves.
3
Filter the mixture to remove the excess solid; the filtrate is the salt solution.
4
Heat the filtrate to evaporate some of the water until crystals start to form on a glass rod.
5
Leave to cool and crystallise, then filter and dry the crystals.
Section 4 of 9Reasoning behind the method
Salt preparation
Why excess solid
- Why excess solid Adding more solid than needed ensures that all the acid reacts, so the solution contains only salt and water.
Why filter
- Why filter The excess insoluble solid can be removed because it does not dissolve.
Why crystallise
- Why crystallise Slow cooling of a hot saturated solution gives larger, purer crystals than boiling to dryness.
Precipitation
- Precipitation An insoluble salt forms immediately as a solid when two solutions are mixed,
- and is collected on filter paper, washed with distilled water and dried.
Section 5 of 9Predicting the salt
| Reactants | Salt formed | Other products |
|---|---|---|
| Zinc oxide + sulfuric acid | Zinc sulfate | Water |
| Magnesium + hydrochloric acid | Magnesium chloride | Hydrogen |
| Calcium carbonate + nitric acid | Calcium nitrate | Water and carbon dioxide |
| Potassium hydroxide + nitric acid | Potassium nitrate | Water |
Section 6 of 9Preparing a soluble salt from an alkali
1
Use a pipette to transfer a measured volume of the alkali into a conical flask and add a few drops of indicator.
2
Add the acid from a burette, swirling, until the indicator just changes colour, and note the volume used.
3
Repeat the same volumes without the indicator, since the indicator would contaminate the salt.
4
Evaporate some of the water from the solution, then leave it to crystallise.
5
Filter off the crystals, wash them with a little cold distilled water and dry them.
Section 7 of 9Choosing between methods
Is the base soluble?
- Is the base soluble?
- If the reactant is an alkali, the excess method does not work, so a titration is used instead.
- If it is an insoluble solid, add an excess and filter.
Is the salt soluble?
- Is the salt soluble? If the salt you want is insoluble, make it by precipitation from two soluble salts, then filter, wash and dry the solid.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Soluble | Able to dissolve in a solvent such as water. |
| Insoluble | Not able to dissolve in a solvent. |
| Filtrate | The liquid that passes through filter paper. |
| Crystallisation | Formation of solid crystals from a saturated solution. |
| Precipitate | An insoluble solid formed when two solutions react. |
| Titration | A method that finds the exact volumes of two solutions that react completely. |
| Burette | A long graduated tube with a tap, used to add measured volumes of liquid. |
Section 9 of 9Exam technique
1
Using an alkali with excess solid; you cannot filter off an excess of something that dissolves.
2
Boiling the solution to dryness, which gives a powder rather than crystals.
3
Forgetting that nitrates are always soluble when predicting precipitates.
4
Not warming the acid or stirring when the reaction is slow.
5
Leaving the indicator in the solution when crystallising, which makes the salt impure.
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